Electrolytic capacitor

ABSTRACT

An electrolytic capacitor having a metal electrode with a dielectric film thereon and an electrolyte which is composed of an organic acid, substances such as p-quinone and manganese dioxide as an ionization substance and highly dielectric solvents such as dimethyl formamide and polyhydric alcohol. A good conductivity and a large film recovery capability are achieved with this composition electrolyte.

United States Patent lnventor Susumu Yoshimura Kadoma, Japan Appl. No.835,927

Filed June 24, 1969 Patented Sept. 2 l, 1971 Assignee MatsushitaElectric Industrial Company,

Limited Osaka, Japan Priority July 2, 1968, Sept. 19, 1968, Sept. 19,

Japan 43-46684, 43-68993 and 43-68994 ELECTROLYTIC CAPACITOR 1 Claim, 2Drawing Figs.

US. Cl. 252/622, 317/230 Int. Cl H01g 9/02 Field of Search 317/230,

Primary Examiner.iames D. Kallam Att0rneyJohn Lezdey ABSTRACT: Anelectrolytic capacitor having a metal electrode with a dielectric filmthereon and an electrolyte vvhich is composed of an organic acid,substances such as p-quinone and manganese dioxide as an ionizationsubstance and highly dielectric solvents such as dimethyl formamide andpolyhydric alcohol.

A good conductivity and a large film recovery capability are achievedwith this composition electrolyte.

ELECTROLYTIC CAPACITOR This invention relates to an electrolyticcapacitor which has an improved performance for a wide range oftemperature, and more particularly to improvements of the electrolytecomposition, according to which a controlled and stable electricalresistance and a high film-reforming capability of the electrolyte arerealized by means of oxidizing agents added to the conventionalnonaqueous, organic electrolytes.

Main advantages of electrolytic capacitors are a largecapacitance-to-volume ratio and a low cost per microfarad ofcapacitance. These use aluminum or tantalum anodes on which a thindielectric film is formed anodically. The presence of an electrolyte isessential for the application of anodic oxide to a capacitor device.There are two types of electrolytic capacitors specified by theelectrolyte adopted, (l) a dryor wet-electrolytic capacitor, and (2) asolid-electrolyte capacilot.

The electrolyte of the best quality used in the conventional dryelectrolytic capacitor comprises a mixture of solvents, dimethylformamide and ethylene glycol, and an ionogen, such as ammonium picrate,dissolved in it. The electrolyte of such type has an electric resistanceof approximate ohmcm. and also has an advantage that it does neitherdeteriorate nor produce a gas at an elevated temperature over 100 C.,while it has disadvantages that it has a coloration and also has aproperty that its resistance gradually increases due to the reduction ofthe picrate ion. Another essential disadvantage of such organicelectrolyte is the inappropriateness for use at high working voltages,because of its low sparking voltage of about 30-40 volts.

The electrolyte of the electrolytic capacitor has preferably theproperties of (l) a recovery capability against a partial breakdown of afilm, (2) as high conductivity as possible to prevent large dielectricloss of the capacitor, and (3) as low water content as possible in orderto prevent the deterioration of the anodic film of the capacitor.

This invention has, therefore, an object of providing an improvedelectrolyte for an electrolytic capacitor which has a conductivity ofthe same or higher order as compared to the conventional one andextremely high film-reforming capability by adding a predeterminedamount of p-quinone or manganese dioxide to the electrolyte soluted withan organic acid into a mixture made with high dielectric solvents ofdimethyl formamide and the polyhydric alcohol group, such as ethyleneglycol.

v The features and advantages of the present invention will becomeapparent from the following description used in conjunction with theaccompanying drawing, in which:

FIG. I is a perspective view of an electrolytic capacitor of roll typeused with an electrolyte in accordance with the present invention; and

FIG. 2 is a graphical representation of the relationship between thecurrent and the time of a formation curve of the electrolyte used in thecapacitor according to the present invention.

The electrolytic capacitor 10 shown in FIG. 1 comprises a positive foil11 made of a filmforming metal such as tantalum, aluminum, etc., whichhas an oxide film formed thereon after being etched with a procedureknown per se, a negative foil 12 made of a metal, a spacer 13 disposedbetween the positive and negative foils 11 and 12 and immersed with acomposition of an electrolyte, a positive external lead 14 connectedwith the positive foil 11, and a negative external lead 15 connectedwith the negative foil 12.

The assembly of such a capacitor is immersed in the composition of theelectrolyte, thereafter this is put into a sealing container (not shown)and then external terminals are so provided as to connect the positiveand negative foils 11 and 12 therewith, respectively.

According to the present invention, the spacer 13 is immersed with aparticular solution of a predetermined organic acid with p-quinone ormanganese dioxide as an ionization substance and a mixture of highdielectric solvents of dimethyl formamide and the polyhydric alcoholgroup, such as ethylene glycol. This electrolyte composition provideshigh conductivity and film-forming capability due to the interactions ofmolecules contained in the substances.

The solution of dimethyl formamide soluted with p-quinone alone has aspecific resistance of 100,000 ohm-cm. or larger and does not exhibit afilm-forming property against the valve metals. On the other hand,several types of organic acids, such as picric acid, salicylic acid,acethylsalicylic acid, benzoic acid, etc., have some extent offilm-forming capacity against the aforementioned metals and relativelylow electrical resistance. They, however, are not suitable for theelectrolytes of an electrolytic capacitor due to their incompletecharacteristics in both resistance and film-forming capacity.

When p-quinone or manganese dioxide is soluted in the solutioncontaining one of the organic acid described above and a suitablesolvent, there realizes a high conductivity and film-forming capability,which enables those organic solutions all suitable for use inelectrolytic capacitor devices. An alu minum foil is anodized atconstant current in an organic electrolyte, and the electrolyticefficiency and the sparking voltage are measured. Electrolytes withoutp-quinone show a low electrolytic efficiency and low sparking voltage ofat most 50 volts, while those containing p-quinone or manganese dioxidegive an enhanced efficiency and sparking voltage.

The mechanism of these phenomena is not yet known, but some comments aredue on the basis of present informations. p-quinone are known to have astrong oxidizability against the other organic compounds. The effect ofthe p-quinone upon the picrate-based electrolyte can be explained on thebasis of the strong oxidizability of the former, and more particularlyof the fact that the activity of the solution is maintained with the aidof p-quinone even if the picrate ions are converted into a reduced formduring the lifetime of the electrolytic capacitor. in short, p-quinoneoxidizes the reduced form of organic ionogen back to the active acidform and then the resistivity and the film-forming capability is notchanged for a wide range of temperature and in the long period ofworking condition.

The aforementioned manganese dioxide or Mn ion has strong oxidizabilityagainst the organic substances known per se. If the manganese ion existsin the solution of the organic substance, the latter may sometimes beconverted into a plastic state as a result of excess oxidation. On theother hand, if aluminum, tantalum, etc., are oxidized by the organicelectrolyte, the oxidizing action is sometimes stopped on its way. Thisphenomenon occurs since the electrolyte has a weak oxidizability, thereduction of the organic ionization substance and the production ofhydrogen has occurred before the metal is oxidized. If a small amount ofmanganese dioxide such as, for example, less than 10 percent is presentin the solution, l the formation speed increases sharply and (2) thespark voltage increases to volts because, since manganese dioxide or Mnion exists, the host organic ion is always present as an oxidizingagent.

Salicylic acid and benzenesulfonic acid are particularly preferable forthe electrolyte together with manganese dioxide because they do notcause coloration of the electrolyte like picric acid in the conventionalelectrolyte. Further, manganese dioxide acts as a water absorbing agentwith the result that it completely absorbs the water contained in theelectrolyte.

The solvent of the electrolyte may be dimethyl formamide in such a case,but it is preferably mixed with a polyhydric alcohol such as ethyleneglycol so as to improve its quality at a low temperature and is alsopreferably mixed with phosphorus pentoxide in order to remove the watercontent contained in the electrolyte known per se.

The aforementioned ammonium picrate has a good conductivity and aconsiderable film-forming property against aluminum. For example, thesolution of 250 grammes ammonium picrate dissolved in 1,000 grammesdimethyl formamide has an electric conductivity of l.4 l 0' Qcm.

This invention will further become apparent from the following Examples:

EXAMPLE I This example uses picric acid.

An electrolyte containing 20 percent by weight p-quinone and 60 percentbyweight dimethyl formamide has a specific resistance of 100,000 ohm-cm.An addition of 1 percent by weight picric acid to the solution reducesthe specific resistance to 100 -1,000 ohm-cm. The addition results inlarge increases in oxidizability of aluminum and tantalum by theelectrolyte. Thus the addition of 1 percent by weight picric acid to theelectrolyte causes a l-100-fold decrease in the specific resistance andcorrespondingly a 10-100-fold increase in oxidizing power. This isparticularly effective for reformation of the oxide film with the resultthat a leakage current is lessened by approximately two orders withrespect to the electrolyte with the picric acid alone.

FIG. 2 shows an example of a comparison of formation curves ofelectrolytes one having 10 percent picric acid and 90 percent dimethylformamide and the other having l0 percent p-quinone, 5 percent picricacid and 85 percent dimethyl formamide.

It is understood from the graph shown in FIG. 2 that the current withthe electrolyte containing p-quinone and picric acid decreases sharplydue to the formation of oxide film.

It is also understood that although p-quinone is a strong oxidant, knownper se, the oxidizing action of p-quinone alone is not sufficient tooxidize aluminum and tantalum. However, if picric acid has been added atany one time to p-quinone, the solution becomes an extremely powerfuloxidant. This is considered to be a result of the formation of anoxidation-reduction reaction system in which the relatively small amountof picric acid carries out a continuous oxidation of the metal by beingitself reoxidized by p-quinone after reduction.

The leak current of the electrolytic capacitor having a thus improvedelectrolyte decreases to less than l0 amperes. Since only one-tenth ofthe amount of picric acid used in the conventional electrolyte isrequired, the coloration problem is also solved.

EXAMPLE II In this example a solution of 250 grammes ammonium picrate in1,000 grammes dimethyl formamide is used.

When aluminum was oxidized using this solution at a constant currentformation, the voltage increased at a velocity of 5 volts per minuterelative to a current of 10 milliamperes per square centimeter. Anaddition of 50-l00 grammes p-quinone to the solution increased itsoxidizing power sharply. The velocity of the voltage increase multiplied5-l0-fold at the same current density as aforementioned.

The electrolyte thus obtained has superior operational characteristicsparticularly at a high temperature, as compared to the priorelectrolyte. lt does not produce any gas, nor deteriorate, and inaddition it maintains a constant capacitance with respect to time.

In order to actually construct the electrolytic capacitor so as toimprove its property at a high temperature known per se, polyhydricalcohol such as ethylene glycol is preferably mixed therewith, and aphosphorus oxide preferably mixed therewith in order to remove its watercontent.

EXAMPLE III This example uses an electrolyte having an organicsubstance, other than ammonium picrate with high electric conductivitybut low oxidizing ability such as, for example ammonium salicylate andammonium benzenesulfonate.

Ammonium salicylate and ammonium benzenesulfonate have an electricresistivity of approximately 10 ohm-cm. lt was impossible to increasethe voltage over 15 volts at a rate of 10 milliamperes per squarecentimeter.

When 50-100 grammes p-quinone (approximate l0 percent) was applied tothis solution similarly to the Example II, the conductivity of thesolution increased up to 20-30 percent at the maximum while theoxidizing ability thereof increased sharply in such a manner that itsformation speed grew 5 volts per minute and accordingly its maximumvoltage or spark voltage increased up to volts.

EXAMPLE IV This example uses an electrolyte having (1) picric acid orammonium picrate, (2) salicylic acid or ammonium salicylate, (3)benzenesulfonic acid or ammonium benzenesulfonate as an organic ionogendissolved in ethylene glycol and dimethyl formamide with 5-10 percentmanganese dioxide added.

In case of using (I) particularly using picric acid alone, the increasedvoltage stopped at 20 volts, but when manganese dioxide was addedthereto the formation speed of 10 volts per minute and spark voltage ofvolts were obtained.

In case of using ammonium picrate, the formation speed of the solutionincreased S-fold.

Although the content of water plays an important role in theabove-mentioned examples, the feature of this invention is that thenormal organic solution can be activated by means of p-quinone and MnOdissolved in it, making the water contact as small as possible.

I claim:

1. A nonaqueous electrolyte for an electrolytic capacitor consisting ofa mixture of polyhydric alcohols and amides, an oxidizing agent selectedfrom the group consisting of p-quim one and manganese dioxide, and anionogen selected from the group consisting of picric acid, salicylicacid, acetylsalicylic acid, benzoic acid and the salts thereof.

